In conventional water treatment using activated sludge, solid-liquid separation of the activated sludge mixed liquor has been required to obtain clean treated water. To this end, it is common to use a strategy in which an activated sludge mixed liquor is introduced into a sedimentation tank to thereby deposit the sludge by the action of gravity, while the supernatant is discharged as treated water from the sedimentation tank. In this case, the sedimentation tank is required to have a settling area large enough and a retention time long enough to deposit activated sludge, thus causing increases in the size and setting volume of the entire treatment system. Moreover, in a case where activated sludge is difficult to deposit by reason of bulking or other causes, the sludge overflows from the sedimentation tank, which leads to reduced quality of the treated water.
In biological nitrogen removal through denitrification and nitrification processes, the nitrified fluid is circulated from nitrification tank to denitrification tank, whereby NOx—N (nitrate nitrogen and nitrite nitrogen) in the nitrified fluid is reduced to a nitrogen gas by the action of denitrifying bacteria present in activated sludge. During this step, a BOD source in raw feed water is used as a hydrogen donor for denitrification. In such a denitrification/nitrification process, the circulating volume from the nitrification tank to the denitrification tank should be increased to achieve high removal of nitrogen. However, when the circulating volume from the nitrification tank is increased for sludge rich in dissolved oxygen, a further problem arises because the introduction of dissolved oxygen causes a reduction in denitrification performance of the denitrification tank to be used under anaerobic conditions.
On the other hand, a strategy using membrane separation instead of a sedimentation tank is conventionally used to conduct solid-liquid separation of activated sludge. In this case, it is common to use a microfiltration membrane or an ultrafiltration membrane as a membrane for solid-liquid separation. However, this strategy requires a suction/pressure pump as a filtration and separation means. Since filtration is usually accomplished under a pressure ranging from several tens of kPa to several hundreds of kPa, a high pump power is required and leads to an increase in running costs. Membrane separation enables the provision of clear treated water completely free from SS, but permeation flux is low, and it requires periodic chemical washing of the membrane in order to prevent membrane pollution.
In recent years, as an alternative to using a sedimentation tank, another strategy for solid-liquid separation of an activated sludge mixed liquor has been proposed in which a filter material composed of a water-permeable sheet such as woven or nonwoven fabrics, or metallic net materials is immersed in an aeration tank to form a secondary deposition layer of sludge particles per se on the surface of the filter, which is then used as a filtration layer to obtain clear filtered water at low hydraulic head pressure. This strategy is called “dynamic filtration.” Such a filter material composed of a water-permeable sheet allows sludge particles to pass through itself, while a cross-flow of the activated sludge mixed liquor generated on the filter surface causes the formation of a secondary deposition layer of sludge flocs on the water-permeable sheet. This sludge layer serves as a filtration layer (i.e., a dynamic filtration layer) so as to achieve solid-liquid separation of sludge and SS contained in feed water. Since the thickness of such a dynamic filtration layer increases over the course of filtration time, the thickened layer results in increased filtration resistance and hence reduced filtration flux. In such a case, aeration is performed through an air-diffusing pipe provided below the filter to remove the dynamic filtration layer of sludge formed on the filter surface, followed by formation of a new dynamic filtration layer to ensure stable filtration flux.
In applying this dynamic filtration technique to the denitrification/nitrification process, there are two possible embodiments, one of which employs a dynamic filter module placed in a nitrification tank, and the other of which employs a dynamic filter module placed in a solid-liquid separation tank provided separately from denitrification and nitrification tanks. However, in both embodiments, the circulating volume from the nitrification tank to the denitrification tank should be increased to achieve high removal of nitrogen; and hence, as in the case of conventional treatment, a further problem arises because the introduction of a nitrified fluid rich in dissolved oxygen causes a reduction in denitrification performance of the denitrification tank.
The present invention overcomes mutually contradictory problems associated with a system for organic wastewater treatment through denitrification and nitrification processes, as stated above, to the effect that when the circulating volume from the nitrification tank to the denitrification tank is increased to achieve high removal of nitrogen, the introduction of a nitrified fluid rich in dissolved oxygen causes a reduction in denitrification performance of the denitrification tank. Thus, the object of the present invention is to provide a system and method for organic wastewater treatment through denitrification and nitrification processes, which system and method achieve high removal of nitrogen without reducing the denitrification performance of denitrification tanks.